multiple producer, multiple consumer, uses pthreads for locking

does not try to get fancy with lockless design, just a simple queue

requires dynamic memory allocation - like I said, just a simple/basic queue.

Each queue is represented by a single sq_t struct. The queue contains a number of individual elements, each represented by an sq_elem_t struct. Add to the queue with sq_push(), remove from the queue with sq_pop(). If you're interested in waiting for data to be pushed to the queue, create a condition var and call sq_add_listener(). You can then use pthread_cond_wait() or pthread_cond_timedwait() and your thread will be awoken when new data is pushed.

Push data to multiple queues by using sq_publish(); it takes an sq_list_t of queues to push() to and a sq_elem_t that will be pushed to all queues on the list, each being entirely independent from its siblings.

Queue flags and element flags are described below, but some notes:

• SQ_FLAG_VOLATILE - if an element has this flag, it means that the data pointer will not stick around. push() will allocate memory and copy the data to the new buffer, and it also means that when you pop(), you must use/copy the data before you free() the element, because the data was allocated along with the element struct itself.

• SQ_FLAG_FREE - if a pop()'d element has this flag then the data pointer must be explicitly free()'d when you're done with the element.

Some of the queue flags are copied into the element flags when you retrieve one with pop()

• SQ_FLAG_OVERRUN - this means one or more push() operations on this queue have failed before the pop() call, so there has been data loss.

if SQ_FLAG_NOWAIT is passed to sq_init(), then (almost) all lock calls can fail and the various sq_*() functions might return SQ_ERR_WOULDBLOCK. This isn't an error so much as an indication that the sq_*() call must be retried. Similar to O_NONBLOCK for the POSIX read() and write() functions.

This repo contains the library in sq.c and sq.h, an implementation of the pthread_barrier API (since OSX doesn't have it), and a stupid/simple demo made up of main.c, t.h and three thread files, t1.c, t2.c and t3.c. You should be able to build by running make.